Method for controlling spark for particulate filter regenerating

ABSTRACT

A system for filtering and oxidizing particulate matter produced by a gasoline direct injection engine is disclosed. In one embodiment, engine spark is controlled such that soot held by a particulate filter may be oxidized even during low engine loads.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from U.S. Provisional PatentApplication No. 61/246,939, entitled “PARTICULATE FILTER SYSTEM ANDMETHOD FOR REGENERATING,” filed Sep. 29, 2009, the disclosure of whichis hereby incorporated by reference in its entirety and for allpurposes.

TECHNICAL FIELD

The present application relates to the field of automotive emissioncontrol systems and methods.

BACKGROUND AND SUMMARY

Direct injection gasoline engines offer improved efficiency because fuelinjected directly into a cylinder can reduce cylinder chargetemperature. As a result, additional air may enter a cylinder ascompared to an equivalent cylinder that has port injected fuel.Consequently, engine power and efficiency may be improved. In addition,direct injection gasoline engines may exhibit improved transient fuelcontrol because there is less tendency for fuel to collect at a cylinderintake port of a direct injection engine than for a port fuel injectionengine. However, direct injection engines may generate soot at higherengine speed and load conditions because there is less time available toatomize fuel in the cylinder. As a result, it may be useful toincorporate a particulate filter in the exhaust system of a directinjection engine. Gasoline engines include those engines fueled by puregasoline or mixtures of gasoline and other fuels such as alcohols.Further, other fuels used in spark ignited engines are also includedsuch as liquid propane gas (LPG) or compressed natural gas (CNG).

In U.S. Patent Application 2009/0193796 a system for treating exhaustgases of a gasoline engine is presented. In several embodiments, athree-way catalyst is followed by a particulate filter. The particulatefilter may be coated with various combinations of platinum, palladium,and rhodium. The coated particulate filter may assist in the oxidationof soot that is held by the particulate filter. It may be beneficial tofilter gasoline engine emissions with a particulate filter, but overtime, a particulate filter may accumulate an amount of soot to theextent that it reduces engine efficiency by increasing backpressure inthe exhaust system. The reference appears to provide little direction asto how to remove soot from a particulate filter. Therefore, the systemdescribed in the reference may cause engine performance to degrade overtime. In addition, the three-way catalysts described in the referenceoperate at higher efficiencies when gases entering the three-waycatalyst are near stoichiometric conditions. However, there may be someengine operating conditions where the temperature of the particulatefilter is less than desired to achieve a desired rate of soot oxidation.The reference appears to offer little direction for overcoming lowparticulate filter temperatures.

The inventors herein have developed a method for controlling a sparkignited engine having a particulate filter, comprising: retarding aspark angle at which spark is delivered to at least one cylinder of aspark ignited engine when an amount of soot held by a particulate filterexceeds a threshold and when engine load is less than a threshold level.

Oxidation of soot held by a particulate filter can be improved when thetemperature of a particulate filter is regulated by retarding sparktiming from minimum spark for best engine torque (MBT). For example,when an engine is idling at a low speed and low load, the engine may notgenerate sufficient heat to achieve a desired rate of soot oxidation forsoot held by a particulate filter. However, in one embodiment of thepresent description, the crankshaft angle at which spark is delivered toa cylinder can be retarded so that the cylinder air-fuel mixturecombusts later in the cylinder cycle, thereby reducing engine work andincreasing an amount of heat discharged from the cylinder to the exhaustsystem. In this way, engine spark can be adjusted so that additionalheat is provided to a particulate filter in the engine exhaust system.Thus, the particulate filter temperature can be increased to improve arate of soot oxidation.

The present description may provide several advantages. Specifically,the description provides a method for increasing a rate of sootoxidation for soot held by a particulate filter. Further, the presentmethod is responsive to driver torque demand so that engine torqueincreases as driver demand torque increases. In addition, the presentmethod can reduce the time to regenerate a particulate filter becauseregeneration may be possible even during low engine loads.

The above advantages and other advantages, and features of the presentdescription will be readily apparent from the following DetailedDescription when taken alone or in connection with the accompanyingdrawings.

It should be understood that the summary above is provided to introducein simplified form a selection of concepts that are further described inthe detailed description. It is not meant to identify key or essentialfeatures of the claimed subject matter, the scope of which is defineduniquely by the claims that follow the detailed description.Furthermore, the claimed subject matter is not limited toimplementations that solve any disadvantages noted above or in any partof this disclosure.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows a schematic view of an exemplary embodiment of a gasolinedirect injection engine;

FIG. 2 a shows a schematic of an example exhaust system configuration;

FIG. 2 b shows a schematic of an example exhaust system configuration;

FIG. 2 c shows a schematic of an example exhaust system configuration;

FIG. 3 shows a schematic of an example exhaust system configuration;

FIG. 4 shows a flow chart of part of a fuel control method to regeneratea particulate filter for a gasoline engine;

FIG. 5 shows a flow chart of the remaining part of a fuel control methodshown in FIG. 4;

FIG. 6 shows a flow chart of a method for increasing the temperature ofa particulate filter for a gasoline engine;

FIG. 7 shows a flow chart of a method for regenerating a particulatefilter while operating a gasoline engine in a deceleration fuel cut-outmode or in a variable displacement mode;

FIG. 8 shows a flow chart of a method for operating a gasoline enginewhile regenerating a particulate filter; and

FIG. 9 shows an example plot of cylinder air-fuel adjustments andexhaust gas oxygen concentration downstream of a particulate filter.

DETAILED DESCRIPTION OF THE DEPICTED EMBODIMENTS

FIG. 1 shows an exemplary embodiment of a gasoline direct injectionengine system generally at 10. Specifically, internal combustion engine10 comprises a plurality of cylinders, one cylinder of which is shown inFIG. 1. Engine 10 is controlled by electronic engine controller 12.Engine 10 includes combustion chamber 14 and cylinder walls 16 withpiston 18 positioned therein and connected to crankshaft 20. Combustionchamber 14 communicates with an intake manifold 22 and an exhaustmanifold 24 via respective intake valve 26 and exhaust valve 28.

Intake manifold 22 communicates with throttle body 30 via throttle plate32. In one embodiment, an electronically controlled throttle can beused. In one embodiment, the throttle is electronically controlled toperiodically, or continuously, maintain a specified vacuum level inintake manifold 22. Note that throttle body 30 and throttle plate 32 maybe located at a location downstream of compression device 90 in someapplications. Alternatively, throttle body 30 and throttle plate 32 maybe omitted.

Combustion chamber 14 is also shown having fuel injector 37 coupledthereto for delivering fuel in proportion to the pulse width of signal(fpw) from controller 12. Fuel is delivered to fuel injector 37 by aconventional fuel system (not shown) including a fuel tank, fuel pump,and fuel rail (not shown). In the case of direct injection engines, asshown in FIG. 1, a high pressure fuel system is used such as a commonrail system.

Spark plug 34 provides an ignition source for the contents of combustionchamber 14. Energy for creating a spark is provided by ignition system35. Controller 12 adjusts the charging of ignition coils that providevoltage to spark plug 34.

In the depicted embodiment, controller 12 is a conventionalmicrocomputer, and includes a microprocessor unit 40, input/output ports42, electronic memory 44, which may be an electronically programmablememory in this particular example, random access memory 46, and aconventional data bus.

Controller 12 receives various signals from sensors coupled to engine10, including but not limited to: measurements of inducted mass airflow(MAF) from mass airflow sensor 50 coupled to the air filter [A on FIG.1]; engine coolant temperature (ECT) from temperature sensor 52 coupledto cooling jacket 54; a measurement of manifold pressure (MAP) frommanifold pressure sensor 56 coupled to intake manifold 22; a measurementof throttle position (TP) from throttle position sensor 58 coupled tothrottle plate 32; and a profile ignition pickup signal (PIP) from Halleffect (or variable reluctance) sensor 60 coupled to crankshaft 20indicating engine speed.

Engine 10 may include an exhaust gas recirculation (EGR) system to helplower NO_(x) and other emissions. For example, engine 10 may include ahigh pressure EGR system in which exhaust gas is delivered to intakemanifold 22 by a high pressure EGR tube 70 communicating with exhaustmanifold 24 at a location upstream of an exhaust turbine 90 a of acompression device 90, and communicating with intake manifold 22 at alocation downstream of an intake compressor 90 b of compression device90. The depicted high pressure EGR system includes high pressure EGRvalve assembly 72 located in high pressure EGR tube 70. Exhaust gastravels from exhaust manifold 24 first through high pressure EGR valveassembly 72, and then to intake manifold 22. An EGR cooler [shown at Yin FIG. 1] may be located in high pressure EGR tube 70 to coolrecirculated exhaust gases before entering the intake manifold. Coolingis typically done using engine water, but an air-to-air heat exchangermay also be used.

Engine 10 may also include a low pressure EGR system. The depicted lowpressure EGR system includes a low pressure EGR tube 170 communicatingwith exhaust manifold 24 at a location downstream of exhaust turbine 90a, and communicating with intake manifold 22 at a location upstream ofintake compressor 90 b. A low pressure valve assembly 172 is located inlow pressure EGR tube 170. Exhaust gas in the low pressure EGR looptravels from turbine 90 a through a aftertreatment device 82 (forexample, a particulate filter including a three-way catalyst becomprised of a wash coat including platinum, palladium, and rhodium) andaftertreatment device 80 before entering low pressure EGR tube 170.Aftertreatment device 82 processes engine exhaust gases to retain sootand oxidize exhaust gas constituents, for example. Additional exhaustsystem configurations are described in the following description andfigures. A low pressure EGR cooler Ya may be positioned along lowpressure EGR tube 170.

Note that in the context of this description, an aftertreatment devicemay include various types of catalysts, including oxidation catalysts,SCR catalysts, catalyzed particulate filter (e.g., a uniform, zonecoated, or layered catalyzed filter), three-way catalysts, and furtherincludes particulate filters, hydrocarbon traps, and NOx traps but doesnot include sensors and actuators such as oxygen sensors, NOx sensors,or particulate sensors. Some specific examples of aftertreatmentconfigurations may be explicitly referred to by example.

High pressure EGR valve assembly 72 and low pressure EGR valve assembly172 each has a valve (not shown) for controlling a variable arearestriction in high pressure EGR tube 70 and low pressure EGR tube 170,which thereby controls flow of high and low pressure EGR, respectively.

Vacuum regulators 74 and 174 are coupled to high pressure EGR valveassembly 72, low pressure EGR valve assembly 172. Vacuum regulators 74and 174 receive actuation signals from controller 12 for controlling thevalve positions of high pressure EGR valve assembly 72, low pressure EGRvalve assembly 172. In a preferred embodiment, high pressure EGR valveassembly 72 and low pressure EGR valve assembly 172 are vacuum actuatedvalves. However, any type of flow control valve or valves may be usedsuch as, for example, an electrical solenoid powered valve or a steppermotor powered valve.

Compression device 90 may be a turbocharger or any other such device.The depicted compression device 90 has a turbine 90 a coupled in theexhaust manifold 24 and a compressor 90 b coupled in the intake manifold22 via an intercooler [shown at X in FIG. 1], which is typically anair-to-air heat exchanger, but could be water cooled. Turbine 90 a istypically coupled to compressor 90 b via a drive shaft 92. (This couldalso be a sequential turbocharger arrangement, single VGT, twin VGTs, orany other arrangement of turbochargers that could be used).

Concentration of oxygen present in the exhaust system may be assessed byoxygen sensors 175, 178 and 176. Further, additional oxygen sensors (notshown) or fewer oxygen sensors may be placed at various locations in theexhaust system as described herein. Oxygen sensor 175 senses enginefeed-gas oxygen concentration while oxygen sensor 178 senses exhaust gasoxygen downstream of aftertreatment device 82. Oxygen sensors may bewide range sensors having a linearized output or they may be sensorsthat indicate a high gain signal near stoichiometric conditions.

Further, drive pedal 94 is shown along with a driver's foot 95. Pedalposition sensor (pps) 96 measures angular position of the driveractuated pedal. It will be understood that the depicted engine 10 isshown only for the purpose of example and that the systems and methodsdescribed herein may be implemented in or applied to any other suitableengine having any suitable components and/or arrangement of components.

Referring now to FIG. 2 a, a schematic of an example exhaust systemconfiguration is shown. Exhaust system 201 is comprised of anaftertreatment device 230 which is comprised of an oxidation catalystand a particulate filter absent of an oxygen storage medium (e.g.,ceria). Alternatively, in some applications aftertreatment device 230may be comprised of a particulate filter or a uniform, zone coated, orlayered catalyzed particulate filter, the filter absent an oxygenstorage medium. Aftertreatment device 230 is shown at the most upstreamlocation 230, downstream of engine 200. Oxygen sensor 202 is locateddownstream of aftertreatment device 230 and upstream of aftertreatmentdevice 232. Aftertreatment device 232 may be comprised of a three-waycatalyst, for example. Oxygen sensor 204 is located downstream ofaftertreatment device 232.

In the embodiment of FIG. 2 a, oxygen sensor 202 advantageously accountsfor engine exhaust gas oxygen that is consumed when soot is combusted inparticulate filter 230. When particulate filter 230 is below theoxidation temperature of soot, oxygen sensor 202 indicates the engineout feed gas oxygen concentration. On the other hand, oxygen sensor 202indicates the concentration of oxygen entering aftertreatment device 232whether or not soot held by particulate filter 230 is combusted.

By sensing the oxygen in engine exhaust gas, it is possible to determinewhether an engine is combusting a rich or lean air-fuel mixture.Further, by sensing exhaust gases entering an aftertreatment device itis possible to estimate and control the operation of the aftertreatmentdevice. In the particular configuration of FIG. 2 a, oxygen sensor 202provides an indication of oxygen entering aftertreatment device 232.Further, oxygen sensor 202 senses oxygen in engine feed gases when sootheld in aftertreatment device 230 is not being oxidized. On the otherhand, when soot is being oxidized by aftertreatment device 230, noestimate of oxidized soot is needed to determine the amount of oxygenentering downstream aftertreatment device 232. Therefore, the amount offuel delivered to engine 200 can be adjusted so that aftertreatmentdevice is exposed to near stoichiometric exhaust gases without having toestimate how much oxygen is consumed by soot oxidation. For example, ifsoot is being oxidized, and oxygen sensor 202 indicates a lean air-fuelmixture, engine fuel can be increased so that the oxygen concentrationentering the downstream aftertreatment device is at a stoichiometriclevel. Conversely, if soot is being oxidized and oxygen sensor 202indicates a rich air-fuel mixture, engine fuel can be decreased. Thus,when oxygen from the engine exhaust gas participates in oxidation ofsoot held by aftertreatment device 230, engine fuel can be adjusted sothat downstream aftertreatment device 232 is exposed to a desired amountof oxygen.

It should also be noted that oxygen sensor 202 may be used to determinewhether other exhaust gas constituents are increasing or decreasing. Forexample, near stoichiometric conditions, an increasing amount of oxygenmay indicate increasing NOx while a decreasing oxygen concentration mayindicate increased HC and CO emissions.

Downstream oxygen sensor 204 may be used to indicate or infer the stateof aftertreatment device 232. In one example, when oxygen sensor 204indicates a lean condition, an air-fuel mixture supplied to engine 200may be enriched so that aftertreatment device 232 may be brought back tostoichiometric conditions. In another example, when oxygen sensor 204indicates a rich condition, an air-fuel mixture supplied to engine 200may be leaned so that aftertreatment device 232 may be brought back tostoichiometric conditions. In this way, an air-fuel mixture supplied toan engine can be adjusted to improve and account for the performance ofan aftertreament device (e.g., a particulate filter) located upstream inan exhaust system while also maintaining efficient operation of adownstream aftertreatment device (e.g., a three-way catalyst).

Referring now to FIG. 2 b, a schematic of an example exhaust systemconfiguration is shown. Upstream oxygen sensor 206 directly sensesengine feed gases from engine 200. Farthest upstream aftertreatmentdevice 240 may be comprised of a particulate filter and a three-waycatalyst. Oxygen sensor 208 senses exhaust gases that have beenprocessed by aftertreatment device 240. By providing an oxygen sensordownstream of aftertreatment device 240, advantages may be realized overa system that provides a sole oxygen sensor at 206 or over a system thatprovides a sole oxygen sensor at 208. For example, engine feed gasemissions may be directly sensed by oxygen sensor 206, while oxygenutilized or stored in the three-way catalyst portion of aftertreatmentdevice 240 is observable by oxygen sensor 208. In addition, oxygensensor 208 senses the reduction in exhaust gas oxygen when soot iscombusted in the particulate filter portion of aftertreatment device240. As a result, the output of sensors 206 and 208 can be compared todetermine when the catalyst portion of the aftertreatment device lightsoff (e.g. light off may be indicated by the catalysts ability convertoxygen) and when oxidation of soot in the particulate filter begins. Forexample, the oxygen concentration passing through the upstreamaftertreatment device can be subtracted from the amount of oxygenentering the upstream aftertreatment device. When the oxygenconcentration deviates from a base level of oxygen usage, it can bedetermined that the catalyst has been activated (e.g., there is catalystlight off) or that soot is being combusted within the particulatefilter. Since the catalyst begins to activate at lower temperatures thanthe temperature at which soot begins to oxidize, it is possible tomonitor oxygen concentrations upstream and downstream of a device thatacts as a catalyst and particulate filter and determine when catalystlight off occurs and when soot oxidation begins. For example, within afirst temperature range of the particulate filter, the oxygenconcentration downstream of an aftertreatment device can be subtractedfrom the oxygen concentration upstream of the aftertreatment device, andthe difference can indicate catalyst light off by sensed oxygen storage.Within a second temperature range, higher than the first temperaturerange, the oxygen concentration downstream of an aftertreatment devicecan be subtracted from the oxygen concentration upstream of theaftertreatment device, and the difference can indicate when soot beginsto oxidize within an aftertreatment device.

Downstream aftertreatment device 242 may be comprised of a three-waycatalyst in the illustrated configuration. And, downstream oxygen sensor212 may be used to indicate the state of downstream aftertreatmentdevice 242. Further, the combination of oxygen sensors 208 and 212 mayprovide even more information as to the state of aftertreatment device242. For example, the output of oxygen sensor 212 can be subtracted fromthe output of oxygen sensor 208 to determine the oxygen storage capacityof aftertreatment device 242. In particular, the difference in oxygensensed by oxygen sensor 208 and oxygen sensor 212 when the state ofaftertreatment device 242 transitions from rich to lean, the differencein oxygen concentration sensed by oxygen sensors 212 and 208 provides anindication of the oxygen storage capacity of aftertreatment device 242.

In one embodiment, portions of soot held by aftertreatment device 240can be oxidized by repeatedly cycling aftertreatment device 240 betweenrich and lean exhaust gas conditions while oxygen in aftertreatmentdevice 242 is depleted and restored without substantially depleting orrestoring the total storage capacity of aftertreatment device 242. Forexample, fuel supplied to an engine can be modulated about stoichiometryso that at least a cylinder of the engine combusts an air-fuel mixturethat is rich or lean of stoichiometry. The frequency, duty cycle, anddegree of richness or leanness can be varied to modulate the combustedair-fuel mixture, thereby modulating the exhaust gas oxygenconcentration. The configuration of FIG. 2 b allows the state ofaftertreatment device 240 to change between an oxygen concentration inexcess of stoichiometric conditions and an oxygen concentration that isless than stoichiometric conditions. At the same time, the state ofaftertreatment device 242 can be monitored so that fuel supplied to theengine is adjusted such that oxygen storage capacity of aftertreatmentdevice 242 is not substantially depleted or filled. For example, theoxygen storage amount can be held near 50% of the oxygen storagecapacity of aftertreatment device 242; or the oxygen storage amount canbe held in a range of 20%-80%, preferably 40-60% of the oxygen storagecapacity.

In another embodiment, oxygen sensor 208 may be removed from the systemof FIG. 2 b. If oxygen sensor 208 is removed, engine fuel adjustmentsmay be based on oxygen sensors 206 and 212. In one embodiment the amountof oxygen entering aftertreatment device 242 may be estimated by a modelthat estimates soot accumulation and soot oxidation. Soot accumulationmay be modeled as a mass from empirically determined test results. Forexample, the amount of soot expelled by an engine at different enginespeeds and loads can be stored in a table or function. When the engineis operated, the table may be interrogated based on present engine speedand load to determine the amount of soot directed to a particulatefilter of the exhaust system. Likewise, the oxidation rate of soot maybe estimated in a similar fashion from engine exhaust oxygenconcentration and particulate filter temperature. By knowing the oxygenconcentration of exhaust gases entering aftertreatment device 240, theoxygen storage capacity of aftertreatment device 240, the oxidation rateof soot of after treatment device 240, and the amount of soot stored byaftertreatment device 240, the amount oxygen entering aftertreatmentdevice 242 can be estimated. When the amount of estimated oxygen storedin aftertreatment device 242 is below a threshold or exceeds athreshold, engine fuel may be adjusted rich or lean to returnaftertreatment device 242 to a desired amount of stored oxygen.

Oxygen sensor 212 provides oxygen concentration information fromdownstream of aftertreatment device 242 so that engine fuel may beadjusted in response to an observable oxygen concentration. For example,if oxygen sensor 212 indicates a lean condition, fuel is increased tothe engine to reduce oxygen in the exhaust gases. If oxygen sensor 212indicates a rich condition, fuel is decreased to the engine to increaseoxygen in the exhaust gases.

Referring now to FIG. 2 c, a schematic of an example exhaust systemconfiguration is shown. Oxygen sensor 220 directly senses exhaust gasesfrom engine 200. Three-way catalyst 250 oxidizes and reduces exhaust gasconstituents before exhaust gases flow to particulate filter 252. Oxygensensor 222 senses exhaust gases that have passed through three-waycatalyst 250 and particulate filter 252. Three-way catalyst 254 furtherprocesses undesirable exhaust gases that have passed through three-waycatalyst 250 and particulate filter 252. Downstream oxygen sensor 224senses oxygen that has passed through the upstream catalysts andparticulate filter.

The system of FIG. 2 c operates similar to the system shown in FIG. 2 b.However, three-way catalyst 250 and particulate filter 252 are separatecomponents so that the volumes of each component can be varied withoutnecessarily having to vary the volume of the other component. In oneexample, the three-way catalyst volume is less than half the volume ofparticulate filter 252 or three-way catalyst 254. By lowering the volumeof the three-way catalyst, the catalyst may light off faster becauseless mass has to be heated before the three-way catalyst reachesoperating temperature. Oxygen sensor 220 provides the same functions andis utilized in a manner similar to oxygen sensor 206. Oxygen sensor 222provides oxygen concentration information of exhaust gases that havebeen processed by three-way catalyst 250. Oxygen sensor 224 providesoxygen concentration information of exhaust gases that have beenutilized in the oxidation of soot. By providing oxygen sensors upstreamand downstream of the particulate filter, a measured discriminationbetween oxygen storage capacity of the farthest upstream catalyst andthe oxygen utilization during soot oxidation is provided. And finally,oxygen sensor 226 provides the same functions and is utilized in amanner similar to oxygen sensor 212 described above.

Referring now to FIG. 3, a schematic of an example exhaust systemconfiguration is shown. Upstream oxygen sensors 302 and 304 directlysense engine feed gases from different cylinder banks of engine 300.Farthest upstream three-way catalysts 320 and 322 are located upstreamof particulate filter 324. Oxygen sensor 306 senses exhaust gases thathave been processed by three-way catalysts 320 and 322. In particular,exhaust gases from two cylinder banks are combined and delivered toparticulate filter 324 before reaching oxygen sensor 306. Oxygen sensor306 is located downstream of particulate filter 324 and upstream ofthree-way catalyst 326. Oxygen sensor 306 provides an indication ofoxygen concentration that is related to both cylinder banks of engine300. In one example, if oxygen sensor 306 observes a higherconcentration of oxygen present in the exhaust system, the cylinder bankthat indicates a leaner air-fuel mixture being combusted will berichened to bring the exhaust gas concentration closer to astoichiometic mixture. Similar to sensor 224 of FIG. 2 c, downstreamoxygen sensor 308 may be used to adjust the amount of fuel delivered tocylinders of engine 300. In particular, oxygen sensor 308 providesoxygen concentration information from downstream of aftertreatmentdevice 326 so that engine fuel to each cylinder bank may be adjusted inresponse to an observable oxygen concentration by downstream sensor 308.For example, if oxygen sensor 308 indicates a lean condition, fuel isincreased to the engine cylinder bank that exhibits the leanest mixtureas sensed at oxygen sensor 302 or 304. If oxygen sensor 308 indicates arich condition, fuel is decreased to the engine to the engine cylinderbank that exhibits the richest mixture as sensed at oxygen sensor 302 or304.

Referring now to FIG. 4, a flow chart of part of a fuel control methodto regenerate a particulate filter for a gasoline engine is shown. At402, engine operating conditions are determined from sensors andactuators. In one example, routine 400 determines engine temperature,ambient temperature, the pressure drop across a particulate filter oraftertreatment device, time since engine start, engine load, enginetorque demand, temperature of a catalyst downstream of a particulatefilter, engine speed, and amount of air inducted to the engine. In otherexample embodiments, additional or fewer operating conditions may bedetermined based on specific objectives. After determining engineoperating conditions routine 400 proceeds to 404.

At 404, an amount of soot held by a particulate filter as well as a sootoxidation rate are determined. As discussed above, in one embodiment,soot accumulation may be modeled as a mass from empirically determinedtest results. In this embodiment, the amount of soot expelled by anengine at different engine speeds and loads can be stored in a table orfunction. When the engine is operated, the table may be interrogatedbased on present engine speed and load to determine the amount of sootdirected to a particulate filter of the exhaust system. Likewise, theoxidation rate of soot may be estimated in a similar fashion from engineexhaust oxygen concentration and particulate filter temperature. Forexample, a table holding oxidation rates of soot may be indexed byparticulate filter temperature and mass flow rate of oxygen to thefilter. If the oxidation rate of soot exceeds the rate of soot storage,the particulate filter is considered as being regenerated because aportion of the soot storage capacity is being liberated by the oxidationof soot. Routine 400 then proceeds to 406.

At 406, routine 400 judges whether or not to initiate regeneration of aparticulate filter. In one embodiment, routine 400 makes a decisionbased on the pressure drop across a particulate filter. In anotherembodiment, routine 400 may decide to regenerate the particulate filterin response to a model. For example, a soot accumulation model thatestimates the amount of soot produced by an engine may be the basis forregenerating a particulate filter. If the estimated amount of sootexceeds a threshold, particulate filter regeneration is initiated. Onthe other hand, if a pressure across the particulate filter isdetermined from a sensor or an estimating model, particulate filterregeneration may be initiated after the observed or estimated pressureexceeds a threshold.

In addition, other conditions may be included that determine when toregenerate the particulate filter. For example, filter regeneration maynot proceed if engine temperature is above a threshold temperature or ifengine temperature is below a threshold temperature. Further in oneexample, filter regeneration may not proceed if filter temperature isbelow a threshold. However, if soot is accumulated on the filter,controller 12 may elevate the filter temperature by retarding spark andincreasing engine air flow as is described by the description of FIG. 6until a threshold filter temperature is reached. In this example,particulate filter regeneration may proceed after the thresholdtemperature is reached. In still another example, particulate filterregeneration may not proceed for a period of time since engine start.For example, particulate filter regeneration may not be initiated untilenough time for engine speed to stabilize after engine start has passed.In another embodiment, particulate filter regeneration may be initiatedduring deceleration fuel shut-off. In yet another embodiment,particulate filter regeneration may not be initiated unless engine loadis greater than a threshold (for example, engine load may be the desiredengine torque divided by total torque available from the engine; inother applications load may be the cylinder air charge divided by thetotal theoretical cylinder air charge), 0.3 load for example. In anotherexample, particulate filter regeneration may not proceed until acatalyst located downstream of a particulate filter is at a thresholdtemperature.

It should be noted that a particulate filter may be actively orpassively regenerated. During active regeneration engine operatingconditions can be adjusted to intentionally facilitate or improveparticulate filter regeneration. For example, engine spark timing may beadjusted to increase the temperature of a particulate filter to increasesoot oxidation. Conversely, passive particulate filter regeneration ispossible when engine operating conditions cause soot held by theparticulate filter to oxidize without a particulate filter regenerationrequest, for example. In one embodiment, a particulate filter may bepassively regenerated when the engine is operated at higher enginespeeds and loads. The regeneration may be passive even though engine airfuel is adjusted in response to an oxygen concentration in the exhaustsystem, the oxygen concentration influenced by the oxidation ofparticulate matter held by the particulate filter.

If particulate filter regeneration is desired and conditions are met,routine 400 proceeds to 408. Otherwise, routine 400 proceeds to 418.

At 418, routine 400 returns to operating the engine with basestoichiometric fuel control. It should be noted that base fuel controlallows the engine to operate lean or rich of stoichiometry during someconditions. For example, an engine may be operated lean with base fuelcontrol during cold start to reduce hydrocarbon emissions. Conversely,an engine may be operated rich with base fuel control during high loadconditions to reduce the possibility of engine degradation. In addition,the engine may be operated in various cyclic lean and rich conditionsthat preserve the time-averaged net stoichiometric conditions.

At 408, routine 400 judges if the particulate filter is at a temperaturethat supports oxidation of soot and other matter that may be held by aparticulate filter. If routine 400 judges that a particulate filter isat a temperature that supports regeneration and oxidation, routine 400proceeds to 410. Otherwise, routine 400 proceeds to 414.

At 414, routine 400 begins to raise the particulate filter temperatureto promote filter regeneration. In particular, the method describe byFIG. 6 is used to elevate the particulate filter temperature. Routine400 then returns to 408 to judge whether or not the particulate filtertemperature is sufficient to proceed to 410.

At 410, routine 400 judges whether to begin the particulate filterregeneration with products of lean or rich combustion. In oneembodiment, during a first operating condition, regeneration begins byramping fuel from substantially stoichiometric (e.g., ±0.04 lambda wherelambda is air-fuel ratio/air-fuel ratio at stoichiometry) combustion torich combustion. In particular, the engine air-fuel is ramped rich untilsubstantially all oxygen storage (e.g., more than 75% of availableoxygen storage capacity) is depleted in any aftertreatment deviceupstream of and including the particulate filter. Then, the engineair-fuel mixture is driven lean by ramping fuel lean or by a step changein cylinder air-fuel mixture (e.g., jumping from 0.95λ to 1.05λ inresponse to oxygen depletion). By depleting oxygen in upstreamaftertreatment devices, it is possible to increase the oxygen flow rateto the particulate filter while reducing the possibility of oxygenslippage through the particulate filter and oxygen and/or NOx slippagethrough aftertreament devices that are located downstream of theparticulate filter. In this way, the rate of oxidation may be improvedbecause kinetic interaction between soot and oxygen increases withhigher oxygen flow rates. In another embodiment, or during a secondoperating condition different than the first operating condition,regeneration begins by adjusting engine air-fuel mixtures lean ofstoichiometric conditions. In one example, the engine air-fuel isgradually adjusted lean so that oxidation of soot gradually increases.In this way, the rate of oxidation can be controlled so that theparticulate filter temperature gradually increases and so that theengine air-fuel ratio can be used to control the temperature of theparticulate filter. In one example, routine 400 judges whether to startthe oxidation process lean or rich in response to a temperature of theparticulate filter. For example, if the particulate filter temperatureis near the threshold oxidation temperature, routine 400 begins theparticulate filter oxidation process by going rich. On the other hand,if the particulate filter temperature is higher than the thresholdoxidation temperature, routine 400 begins the particulate filteroxidation process by going lean. If routine 400, judges to start theparticulate filter oxidation process by going rich, routine 400 proceedsto 412. Otherwise, routine 400 proceeds to 416. Thus, routine 400provides the ability to always begin the particulate filter oxidationprocess rich or lean. But, routine 400 also provides for beginning theparticulate filter oxidation process rich or lean depending onconditions. For example, under a first condition the particulate filteroxidation process may begin lean or rich, and under a second conditionthe particulate filter oxidation process may begin in the other of stateof rich or lean under a second condition.

At 412, routine 400 begins particulate filter oxidation by rampingengine air-fuel rich until it is determined that exhaust gasesdownstream of the particulate filter contain a threshold amount less ofoxygen than a stoichiometric exhaust gas mixture. In one embodiment, anoxygen sensor downstream of the particulate filter provides data thatindicates when the oxygen upstream of the oxygen sensor is substantiallydepleted. In one embodiment, the extent that the engine air-fuel mixturecan be richened is limited to a threshold amount. After the air-fuelratio of the engine is shifted rich, routine 400 proceeds to theremainder of the routine described by FIG. 5 and routine 500.

At 416, routine 400 begins particulate filter oxidation by rampingengine air-fuel lean until it is determined that exhaust gasesdownstream of the particulate filter contain a threshold amount more ofoxygen than a stoichiometric exhaust gas mixture. In one embodiment, anoxygen sensor downstream of the particulate filter provides data thatindicates when the oxygen begins to break through aftertreatment devicesthat are located upstream of the oxygen sensor. In one embodiment, theextent that the engine air-fuel mixture can be leaned is limited to athreshold amount. After the air-fuel ratio of the engine is shiftedlean, routine 400 proceeds to the remainder of the routine described byFIG. 5 and routine 500.

Referring now to FIG. 5, the remainder of the routine shown in FIG. 4 isshown. At 502, routine 500 judges whether or not there is an oxygensensor located upstream of a catalyst and if the oxygen sensor is theoxygen sensor used to determine engine feed gas oxygen concentration.Routine 500 may judge the location of oxygen sensors based on systemconfiguration information stored in memory of an engine controller, forexample. If routine 500 judges that the farthest upstream oxygen sensoris located upstream of a catalyst, routine 500 proceeds to 504;otherwise, routine 500 proceeds to 506.

At 504, routine 500 accounts for the amount of accumulated soot oxidizedwithin a particulate filter. Specifically, in one embodiment, routine500 adjusts the stoichiometric air-fuel ratio leaner so that the enginefeed gas oxygen concentration indicates a stoichiometric engine air-fuelratio after the engine exhaust gases pass through the particulate filterand a portion of the engine feed gas oxygen oxidizes the soot held by aparticulate filter. Routine 500 then proceeds to 506.

At 506, routine 500 judges whether or not there is a catalyst upstreamof a particulate filter and whether or not the catalyst has oxygenstorage capacity. Alternatively, the catalyst may be included with theparticulate filter. Routine 500 may judge whether or not there is acatalyst upstream of the particulate filter and whether or not thecatalyst has oxygen storage capacity based on system configurationinformation stored in memory of an engine controller, for example. Ifroutine 500 judges that there is a catalyst with oxygen storagecapacity, routine 500 proceeds to 508. If no catalyst exists or if thecatalyst does not comprise an oxygen storage media, routine 500 proceedsto 510.

At 508, routine 500 determines the oxygen storage capacity of theupstream catalyst. In one embodiment, the oxygen storage capacity isdetermined from a table that contains oxygen storage data that may beindexed by catalyst temperature. In addition, the oxygen storagecapacity extracted from the table can be adjusted to account forcatalyst degradation that may occur over time. In one embodiment, theoxygen storage capacity is adjusted based on cycling the catalystbetween lean and rich conditions and detecting when the state of thecatalyst changes by data from oxygen sensors located upstream anddownstream of the catalyst. After determining the oxygen storagecapacity of the catalyst, routine 500 proceeds to 510.

At 510, the engine air-fuel ratio is adjusted to vary the exhaust gasconstituents entering the upstream catalyst if one is present. In oneembodiment, where an upstream oxygen sensor is located between theengine and a catalyst, the upstream oxygen sensor provides feedback ofthe engine feed gas oxygen concentration. Further, the upstream oxygensensor indicates the oxygen concentration that is entering the upstreamcatalyst. By multiplying the oxygen concentration by the mass flow ratethrough the engine, the mass of oxygen entering the upstream catalystmay be determined. In one embodiment, an oxygen sensor located upstreamof a catalyst determines how much oxygen (e.g., the mass of oxygen) isbeing delivered to the catalyst over an interval of time. In oneembodiment, the rate at which oxygen is supplied to the upstreamcatalyst can be adjusted based on operating conditions. For example, therate at which oxygen is delivered to the upstream catalyst andparticulate filter can be increased when the temperature of theparticulate filter exceeds the threshold oxidation temperature by athreshold amount while the particulate filter temperature is below adifferent threshold temperature. When the temperature of the particulatefilter is decreasing or near the threshold oxidation temperature, therate at which oxygen is delivered to the upstream catalyst andparticulate filter can be decreased.

In one embodiment during particulate filter regeneration, fuel suppliedto the engine is controlled by fuel control parameters that aredifferent than fuel control parameters used to control engine fuelingwhen the engine is operated under similar conditions while a particulatefilter is not being regenerated. For example, the rate at which oxygenis delivered to the exhaust system and the extent of the leanness orrichness from stoichiometric conditions can be different when aparticulate filter is being regenerated as compared to when aparticulate filter is not being regenerated, while the engine isoperating at similar operating conditions. In one embodiment, additionaloxygen is added to the exhaust gas constituents by leaning cylinderair-fuel mixture while the particulate filter is being regenerated.

If an upstream catalyst is not present in a particular configuration,the engine air-fuel ratio may be adjusted to promote oxidation of soot.In one embodiment, an oxygen sensor located upstream of a particulatefilter may control the level of oxygen delivered to the particulatefilter. For example, an amount of oxygen in excess of a stoichiometricexhaust gas concentration may be made in response to the amount of sootheld by the particulate filter or in response to the rate of sootoxidation. For higher amounts of soot held by the particulate filter,higher amounts of oxygen may be supplied to the particulate filter. Forlower amounts of soot held by the particulate filter, lower amounts ofoxygen may be supplied to the particulate filter. In this way, theamount of oxygen of engine exhaust gases can be controlled so that theexcess oxygen in the exhaust gas is used to oxidize soot held in theparticulate filter and so that the state of a catalyst that isdownstream of the particulate filter is not disturbed to an extent whereNOx breaks through a catalyst that is downstream of the particulatefilter. FIG. 8 provides more detail as to how engine air-fuel isadjusted during particulate filter regeneration.

At 512, the engine air-fuel ratio is adjusted to vary the exhaust gasconstituents entering the downstream catalyst. In one embodiment, theengine air-fuel ratio determined at 510 is adjusted so that the state ofa catalyst downstream of the particulate filter is changed. For example,the air-fuel mixture of a cylinder can be adjusted leaner or richer thanthe adjustment of air-fuel mixture determined at 510. By varying theengine air-fuel mixture the state of the downstream catalyst is adjustedso that it converts efficiently while in a particulate filterregeneration mode.

The engine air-fuel may be adjusted to control the state of a catalystdownstream of a particulate filter by way of an oxygen sensor locatedupstream of the downstream catalyst, an oxygen sensor located downstreamof the downstream catalyst, or by a combination of the oxygen sensorlocated upstream of the downstream catalyst and the oxygen sensorlocated downstream of the downstream catalyst. In one example, theair-fuel mixture entering a cylinder can be adjusted richer when anoxygen sensor located downstream of the downstream catalyst indicates alean condition. When the oxygen sensor located downstream of thedownstream catalyst indicates a rich condition, the cylinder air-fuelratio may be adjusted leaner. On the other hand, the oxygen sensorlocated upstream of the downstream catalyst can be used to adjust theair-fuel mixture of a cylinder rich when a threshold amount of exhaustgas that is lean has entered the downstream catalyst. When the oxygensensor located upstream of the downstream catalyst indicates that athreshold amount of exhaust gas has entered the downstream catalyst isrich, the cylinder air-fuel mixture can be adjusted lean. In this way,the amount of oxygen present in the downstream catalyst can becontrolled so that HC and CO may be oxidized while NOx is reduced.

At 514, routine 500 can adjust engine cylinder air-fuel mixtures tocontrol the rate of soot oxidation. In one example, oxygen can beintroduced to the particulate filter by way of a lean cylinder air-fuelmixture so that excess oxygen is available at the particulate filter tooxidize soot. If the oxidation rate is higher than desired or if theparticulate filter temperature increases above a threshold temperature,engine cylinder air-fuel mixtures can be enriched so that less oxygen isavailable to participate in the oxidation of soot held by theparticulate filter. The particulate filter temperature may be measuredby a sensor or inferred from engine operating conditions, for example.In addition, the rate that oxygen is delivered to the particulate filtermay be varied depending on operating conditions. For example, if theparticulate filter temperature is higher than a threshold oxidationtemperature but lower than a desired oxidation temperature, then theamount of oxygen supplied to the particulate filter can be increased byleaning cylinder air-fuel mixtures. But, if the particulate filtertemperature is higher than a threshold oxidation temperature, but near adesired oxidation temperature, then the amount of oxygen supplied to theparticulate filter can be decreased by richening cylinder air-fuelmixtures.

At 516, routine 500 determines if the particulate filter has beensufficiently regenerated. In other words, the routine determines if adesired amount of soot held by a particulate filter has been oxidized.Routine 500 judges whether or not filter regeneration is complete or ifconditions for regeneration are no longer present. In one embodiment,regeneration is determined complete when the pressure difference acrossthe particulate filter is less than a predetermined amount. In anotherexample, regeneration is determined as complete when the exhaust gasdownstream of the particulate filter indicates an increase in oxygenconcentration in exhaust gases that pass through the particulate filter.The increased oxygen concentration may be an indicator that soot in thefilter has been oxidized and that the amount of soot is reduced suchthat less oxygen is consumed to oxidize soot remaining in the filter. Ifroutine 500 judges that regeneration is complete, routine 500 proceedsto 518. Otherwise, routine 500 proceeds to 510.

At 518, routine 500 returns fuel control to base fuel control. In oneexample embodiment, fuel is adjusted so that over an interval of time,less oxygen is present in exhaust gases when particulate filterregeneration has stopped as compared to when particulate filterregeneration is ongoing. Of course, many ways are available toaccomplish this result. For example, the amount of time or the number ofcylinder events during which the engine operates lean can be reduced. Inanother example, the extent to which cylinders are operated lean can bereduced. For example, a cylinder may be operated with a stoichiometricair-fuel mixture rather than with a mixture that is lean by 0.5 air-fuelratio. In these ways, the engine air-fuel ratio can be adjusted back tobase fuel where the engine is combusting a substantially stoichiometricair-fuel mixture, for example.

Referring now to FIG. 6, a flow chart of a method for increasing thetemperature of a particulate filter for a gasoline engine is shown. At602, routine 600 judges whether or not a particulate filter is at adesired threshold oxidation temperature. If so, routine 600 proceeds to610 where spark is advanced to minimum spark for best torque (MBT) or toknock limited spark. If the particulate filter is not at a desiredtemperature, routine 600 proceeds to 604. It should be noted that thedesired threshold oxidation temperature may be set above a temperaturewhere soot oxidation begins. For example, a desired thresholdtemperature may be set at 40° C. above the temperature where soot beginsto oxidize.

At 604, routine 600 judges if the engine is operating in a region wherespark retard is desired. In one example, spark may not be retarded whenengine load is above a threshold level. In addition, the threshold maybe varied for different engine speeds. For example, spark may not beretarded at an engine speed of 1200 RPM for engine loads greater than0.6 whereas spark may not be retarded at an engine speed of 5000 RPM forengine loads greater than 0.45. In another embodiment, duringregenerating a particulate filter of a spark ignited engine spark timingof at least one cylinder of an engine can be adjusted to regulate atemperature of said particulate filter above a threshold temperature.Further, spark timing can be advanced in response to an increasingdriver demand torque. For example, if spark is retarded by 10 degrees toincrease a temperature of a particulate filter, the spark can beadvanced as a driver torque demand increases so that the engine producesthe desired torque and so that the engine has a desired torque response.If the driver subsequently lowers the driver demand torque, the sparkmay be retarded as the driver torque demand is lowed so that a desiredparticulate filter temperature is achieved.

If the engine is operating at conditions where it is desirable to retardspark, routine 600 proceeds to 606 where spark is retarded. Otherwise,routine 600 proceeds to 610.

At 606, engine spark is retarded from MBT or knock limited spark. In oneexample, the spark may be gradually retarded over a number of enginecombustion events so that it is less apparent to a vehicle operator. Theamount of spark retard may be empirically determined and stored in atable or function that is indexed by particulate filter temperature,engine speed, and engine load.

At 608, routine 600 increases cylinder air charge so that equivalenttorque can be produced by the engine while spark is being retarded toheat the particulate filter. In one example, the amount of additionalair is stored in a table indexed by spark retard from MBT, engine speed,and engine load. Thus, engine spark advance and engine cylinder airamount are simultaneously adjusted so that the engine will deliver thedesired operator torque while increasing particulate filter temperature.

Referring now to FIG. 7, a flow chart of a method for regenerating aparticulate filter while operating a gasoline engine in a decelerationfuel cut-out (DSFO) mode or while in a variable displacement mode (VDE)is shown. During DSFO fuel to at least one cylinder is cut-out orreduced to a level where combustion is not possible with the lean fuelmixture. During VDE mode the number of active cylinders producing torqueis less than the number of total engine cylinders. Lean mode VDE or DSFOmay be initiated in one embodiment when the temperature of a particulatefilter is greater than a threshold amount and when it is desirable tooperate the engine cylinders lean, for example at 416 of FIG. 4 or at510-514 of FIG. 5. Further, VDE mode may be entered when cylinder loadis low and particulate filter temperature exceeds a threshold amount.

At 702, routine 700 judges whether or not lean particulate filterregeneration is requested or desired. If so, routine 700 proceeds to704. If not, routine 700 proceeds to exit. During lean particulatefilter regeneration, the particulate filter is regenerated by supplyingexcess oxygen to the particulate filter so that soot may be oxidized bythe excess oxygen.

At 704, routine 700 judges whether or not conditions are met for VDE orDSFO particulate filter regeneration. In one example, the engine may beoperating in a predetermined threshold engine speed/load range for VDE.DSFO particulate filter regeneration may be activated when theoperator's foot is off the throttle and while the vehicle is above athreshold speed. If VDE or DSFO conditions are not met routine 700proceeds to 714 where lean particulate filter regeneration may beaccomplished by adjusting engine cylinder air-fuel ratios withoutdeactivating engine cylinders. FIG. 8 provides details on particulatefilter regeneration by this method. If DSFO or VDE lean mode particulatefilter regeneration conditions are met routine 700 proceeds to 706.

At 706, routine 700 judges whether or not the particulate filter is at adesired temperature for regeneration. In one example, the particulatefilter must be above a threshold temperature. The threshold temperaturemay be at or above the temperature at which soot will oxidize. If theparticulate filter is above the threshold temperature, routine 700proceeds to 712. If the particulate filter is not above the thresholdtemperature, routine 700 proceeds to 708.

At 708, engine spark is retarded from MBT or knock limited spark. In oneexample, the spark may be gradually retarded over a number of enginecombustion events so that it is less apparent to a vehicle operator. Theamount of spark retard may be empirically determined and stored in atable or function that is indexed by particulate filter temperature,engine speed, and engine load.

At 710, routine 700 increases cylinder air charge so that equivalenttorque can be produced by the engine while spark is being retarded toheat the particulate filter. In one example, the amount of additionalair is stored in a table indexed by spark retard from MBT, engine speed,and engine load. Thus, engine spark advance and engine cylinder airamount are simultaneously adjusted so that the engine will deliver thedesired operator torque while increasing particulate filter temperature.Note that 708 and 710 may not retard spark and increase engine airamount unless engine load is below a threshold level that may be variedfor different engine speeds.

At 712, engine cylinders may be deactivated to support VDE or DSFOmodes. In one example, cylinders are deactivated in response to desiredengine load and engine speed. While in VDE or DSFO the deactivatedcylinders may provide oxygen to the particulate filter by pumping a leanair-fuel mixture through the engine and to the exhaust system.Alternatively, engine cylinders may pump intake system gases through theengine and to the particulate filter. While the deactivated cylindersare pumping oxygen to the particulate filter, active cylinders mayoperate with a rich air-fuel mixture and/or retarded spark timing. If acatalyst is located upstream of the particulate filter the rich cylindermixture and the contents of the inactive cylinder may combine at anupstream catalyst to provide additional heat to increase the particulatefilter temperature. In one embodiment, air-fuel mixtures combusted inactive cylinders may be alternated between lean and rich mixtures. Forexample, a particular cylinder may combust a rich air-fuel mixture forone cylinder cycle and then combust a lean air-fuel mixture for one ormore cylinder cycles. In this way, rich and lean air-fuel mixtures maybe periodically cycled so that the particulate filter is exposed toexcess oxygen while a downstream catalyst is maintained nearstoichiometric conditions.

Referring now to FIG. 8, a flow chart of a method for operating agasoline engine and regenerating a particulate filter is shown. Themethod allows near stoichiometric conditions to be maintained in adownstream catalyst.

At 802, routine 800 judges if particulate filter regeneration iscomplete. If so, routine 800 proceeds to exit. If not, routine 800proceeds to 804. The method described at 516 of FIG. 5 may be used at802 to determine if particulate filter regeneration is complete and hastherefore been omitted for brevity. And, if the method of FIG. 8 is usedat 510 and 512 of FIG. 5, 802 may be omitted as this function isperformed at 516.

At 804, routine 800 determines the oxygen storage capacity of allcatalysts in the exhaust system. In one example, the oxygen storagecapacity may be determined as described above by cycling systemcatalysts between lean and rich conditions and observing the time thatit takes the catalyst to change state. In another example, the oxygenstorage capacity may be determined while cycling system catalystsbetween rich and lean states while recording the mass of oxygendelivered to catalyst (e.g., oxygen concentration multiplied by enginemass flow rate). In yet another embodiment, oxygen storage of eachcatalyst may be stored in a table indexed by catalyst temperature andmodified by observations of switching times between oxygen sensorslocated upstream and downstream of catalysts.

At 806, the amount of soot held by the particulate filter is determined.As described above, the amount of soot may be determined by a pressuredrop measured across the particulate trap. Or alternatively, theaccumulated soot and soot oxidation rate may be determined from a modelthat describes the amount of soot produced by the engine (e.g., a tableindexed by engine speed and load) and the soot oxidation rate (e.g.,soot oxidation rate may be related to particulate filter temperature andto the amount of oxygen available in engine exhaust gases).

At 808, routine 800 determines the amount of oxygen stored in eachcatalyst of the system. As exhaust gases pass through an exhaust system,oxygen can be extracted from the gas and used to oxidize HC or CO atsystem catalysts or in the particulate filter; therefore, routine 800keeps track of where oxygen is stored and used in the exhaust system.For example, in one embodiment, the mass of oxygen contained in enginefeed gases is observed by an upstream oxygen sensor before the exhaustgases pass through a catalyst or particulate filter. As the exhaustgases pass through a catalyst or particulate filter a portion of theoxygen may be used to oxidize HC, CO, and soot. The amount of oxygenconsumed from the exhaust gas can be estimated by multiplying the enginefeed gas oxygen mass by a utilization factor for each catalyst orparticulate filter. The utilization factor for each aftertreatmentdevice may be adjusted based on temperatures and mass flow rates, forexample. Oxygen that does not participate in oxidation and is notobserved at a downstream oxygen sensor can be deemed to be stored in acatalyst. If an oxygen sensor detects an oxygen concentration that isnot expected or inconsistent with an estimated amount of stored oxygenthe oxygen storage capacity of each catalyst can be reset or adjusted.In this way, the amount of oxygen stored in each oxygen storage catalystmay be estimated.

At 810, routine 800 determines a particulate filter temperature. In oneembodiment a temperature sensor may be used to determine particulatefilter temperature. In another embodiment, particulate filtertemperature may be estimated based on engine speed, engine load, enginespark advance, and engine air-fuel mixture. For example, a table ofempirically determined exhaust temperatures may be stored and retrievedat a later time so that an engine controller can estimate particulatefilter temperature.

At 812, routine 800 judges whether or not to increase a rate of sootoxidation. In one embodiment, a desired rate of soot oxidation may bebased on the amount of soot held by a particulate filter and the engineload. For example, if the rate of desired soot oxidation is 0.1 mg/secand the present rate of soot oxidation is 0.05 mg/sec the cylinderair-fuel ratio can be leaned by 0.01λ based on 50 mg of trapped soot. Inanother example, if the rate of desired soot oxidation is 0.1 mg/sec andthe present rate of soot oxidation is 0.05 mg/sec the cylinder air-fuelratio can be leaned by 0.05λ based on 20 mg of trapped soot. Thus,during a first condition a cylinder air-fuel may be adjusted to changethe rate of soot oxidation in response to a first amount of soot held bya particulate filter, and during a second condition a cylinder air-fuelmay be adjusted to change the rate of soot oxidation in response to asecond amount of soot held by the particulate filter. If the estimatedrate of soot oxidation is less than a desired oxidation rate, routine800 proceeds to 814. If the estimated rate of soot oxidation is greaterthan a desired rate, routine 800 proceeds to 826.

At 814, routine 800 judges whether or not the engine cylinder air-fuelmixture is at a lean limit. If so, routine 800 proceeds to 818.Otherwise, routine 800 proceeds to 816 where the cylinder air-fuelmixture is leaned out. The engine or a cylinder may be gradually leanedout at 816 or it may be leaned out in a step wise manner to somepredefined amount. For example, a small amount of oxygen may bedelivered to the exhaust system by leaning a cylinder air-fuel mixturefrom λ=1 to λ=1.01. Over time (e.g., 5 seconds) and over a number ofcombustion events (e.g., 500 events), oxygen is slowly added to theexhaust system. On the other hand, the same amount of oxygen may bedelivered to the exhaust system over a shorter time by leaning acylinder to λ=1.1. The rate of leaning or fuel reduction of a cylindermay be related to the desired oxidation rate or the amount of soot heldby the particulate filter. For example, a cylinder may be moved by0.001λ per minute when a particulate filter is half full and at a rateof 0.002λ per minute when the particulate filter is full. Thus, under afirst condition, fuel to a cylinder may be leaned at a first rate, andunder a second condition fuel to a cylinder may be leaned at a secondrate, the second rate different than the first rate.

At 818, routine 800 judges whether or not a downstream catalyst is at alean limit. In the illustrated configurations of FIG. 2 a-2 c and FIG.3, the downstream catalysts act as a buffer within which exhaust gasconstituents are treated even though excess oxygen is provided to theparticulate filter and upstream catalysts. However, it is desirable tokeep the downstream catalysts above a threshold temperature and in astate where between 20%-80% (preferably between 40-60%) of the oxygenstorage capacity of the catalyst oxygen storage capacity is utilized. Ifthe catalyst temperature falls below the threshold temperature, or ifexcess oxygen is stored in the catalyst tailpipe emissions of HC, CO,and NOx may increase. Therefore, routine 800 judges if the downstreamcatalyst is at the lean limit based on the amount of oxygen capacity ofthe downstream catalyst as well as the amount of oxygen stored in thecatalyst. If the amount of oxygen stored exceeds a threshold amount,routine 800 proceeds to 820. Otherwise, routine 800 proceeds to 824.

At 820, routine 800 ramps engine fuel rich even though a higher rate ofoxidation may be desired. Routine 800 ramps rich so that a downstreamcatalyst may continue to operate efficiently, the fuel is ramped richuntil the downstream catalyst is at the rich limit and then leanoperation for particulate filter soot reduction may be resumed. When theengine expels products of lean combustion, a portion of the oxygen inthe exhaust gases is consumed by oxidizing soot held by the particulatefilter. Therefore, the engine can operate lean for an extended periodsince less oxygen will enter the downstream catalyst.

At 824, routine 800 judges whether or not a downstream catalyst is at arich limit. As mentioned above, it is desirable to keep the downstreamcatalyst in a state where between 20%-80% (preferably between 40%-60%)of the catalyst oxygen storage capacity is utilized. In this state, thecatalyst retains constituents for oxidizing and reducing exhaust gases.Therefore, routine 800 judges if the downstream catalyst is at the richlimit based on the amount of oxygen capacity of the downstream catalystas well as the amount of oxygen stored in the catalyst. If the amount ofoxygen stored in the downstream catalyst is less than a thresholdamount, routine 800 proceeds to 822. Otherwise, routine 800 proceeds to802.

At 822, routine 800 ramps engine fuel lean. Routine 800 ramps lean sothat a downstream catalyst may continue to operate efficiently. In oneexample, the fuel is ramped lean until the desired level of oxygen isstored in the downstream catalyst is reached. During lean operation thefuel may be ramped until a desired air-fuel mixture in the cylinder isreached, then the engine may continue to operate at the lean air-fuelmixture until the downstream catalyst reaches the desired storage levelof oxygen.

At 826, routine 800 judges whether or not to decrease particulate filteroxidation by richening the engine cylinder air-fuel ratio. As describedabove, in one embodiment, a desired rate of soot oxidation may be basedon the amount of soot held by a particulate filter and the engine load.For example, if the rate of desired soot oxidation is 0.05 mg/sec andthe present rate of soot oxidation is 0.1 mg/sec the cylinder air-fuelratio can be richened by 0.02λ based on 5 mg of trapped soot. In anotherexample, if the rate of desired soot oxidation is 0.05 mg/sec and thepresent rate of soot oxidation is 0.15 mg/sec the cylinder air-fuelratio can be richened by 0.05λ based on 2 mg of trapped soot. Thus,during a first condition a cylinder air-fuel may be adjusted to changethe rate of soot oxidation in response to a first amount of soot held bya particulate filter, and during a second condition a cylinder air-fuelmay be adjusted to change the rate of soot oxidation in response to asecond amount of soot held by the particulate filter. If the estimatedrate of soot oxidation is greater than a desired oxidation rate, routine800 proceeds to 828. Otherwise, routine 800 proceeds to 818.

At 828, routine 800 judges whether or not the engine cylinder air-fuelmixture is at a rich limit. If so, routine 800 proceeds to 818.Otherwise, routine 800 proceeds to 830 where the cylinder air-fuelmixture is richened. The engine or a cylinder may be gradually richenedat 830 or it may be richened in a step wise manner to some predefinedamount. For example, a small amount of oxygen may be extracted fromexhaust gases by richening a cylinder air-fuel mixture from λ=1 toλ=0.98. Over time (e.g., 5 seconds) and over a number of combustionevents (e.g., 500 events), oxygen is slowly extracted from exhaustgases. On the other hand, the same amount of oxygen may extracted fromexhaust gases over a shorter time by richening a cylinder to λ=0.9.

At 830, the engine cylinder air-fuel mixture is richened. The engine ora cylinder may be gradually richened at 830 or it may be richened in astep wise manner to some predefined amount. For example, a small amountof oxygen may be removed from exhaust gases by richening a cylinderair-fuel mixture from λ=1 to λ=0.98. Over time (e.g., 5 seconds) andover a number of combustion events (e.g., 500 events), oxygen is removedfrom the aftertreatment devices because the stored oxygen is used tooxidize increasing HC and CO. On the other hand, the same amount ofoxygen may be removed from aftertreatment devices over a shorter time byrichening a cylinder to λ=0.9. The rate of richening of a cylinder maybe related to the desired oxidation rate or the amount of soot held bythe particulate filter similar to that which is described at 816.

In this way, the method of FIG. 8 adjusts a cylinder air-fuel duringregeneration of a particulate filter such that the average or integratedair-fuel mixture over a number of cylinder cycles moves leaner. At thesame time, the concentration of oxygen in exhaust gases downstream ofthe particulate filter are reduced such that the average or integratedexhaust gas mixture over a number of cylinder cycles is as substantiallystoichiometric conditions.

It should be noted that all of routines 4-8 may be executed by a singlecontroller, or alternatively, an engine controller may execute only aportion of methods 4-8. Thus, the routines 4-8 may be employed forvarious system configurations.

Referring now to FIG. 9, example plot of cylinder air-fuel adjustmentsand exhaust gas oxygen concentration downstream of a particulate filteris shown. The top plot shows an example air-fuel ratio of a cylinderover a number of cylinder cycles. The air-fuel oscillates about anx-axis that represents a stoichiometric air-fuel ratio. Time increasesfrom left to right. Before T1 the cylinder air-fuel ratio is symmetricabout stoichiometry and the particulate filter is not being regenerated.Between T1 and T2 particulate filter regeneration commences and thecylinder air-fuel ratio is shifted lean to account for oxygen thatparticipates in the combustion of soot in a particulate filter. Noticethat the entire oscillating air-fuel ratio is shifted lean. Between T2and T3 the cylinder air-fuel ratio is shifted even further lean tofurther increase the rate of oxidation in the particulate filter. Therich side of the air-fuel mixture also increases in an extent ofrichness or leanness of an air-fuel mixture entering a cylinder in orderto keep the rear three-way catalyst balanced and operating efficiently.After T3 the cylinder air-fuel ratio is shift richer and is againsymmetric about stoichiometry when soot oxidation has completed.

It should be noted that the air-fuel depicted in FIG. 9 is merelyprovided for illustration purposes and is not intended to limit thedescription in any way. For example, engine air-fuel may be controlledby a triangular air-fuel distribution or by a stochastic distributionabout stoichiometric conditions. In addition, the duration of air-fuelrichness or leanness as well as the extent of air-fuel leanness orrichness may be adjusted to keep a downstream three-way catalystbalanced to stoichiometric conditions.

The bottom plot illustrates oxygen concentration in the exhaust systemat a location downstream of the particulate filter. Notice that theoxygen concentration stays symmetric about stoichiometry when thecylinder air-fuel is shifted lean when soot is oxidized duringparticulate filter regeneration. As lean exhaust gases pass through theparticulate filter between T1 and T3, oxygen participates in theoxidation of soot to CO and/or CO₂. The partial oxidation of soot to COmay provide a reductant for NOx reduction. As a result, the oxygenconcentration exiting the particulate filter is symmetric aboutstoichiometric conditions. Consequently, stoichiometric conditions aremaintained in a downstream catalyst. In this way, a particulate filterupstream can be regenerated while maintaining stoichiometric conditionsin a downstream catalyst. Thus, a downstream catalyst can efficientlyconvert exhaust gases while oxidizing soot in the particulate filter.

It will be appreciated that the configurations and routines disclosedherein are exemplary in nature, and that these specific embodiments arenot to be considered in a limiting sense, because numerous variationsare possible. For example, the above approaches can be applied to V-6,I-4, I-6, V-12, opposed 4, and other engine types.

The subject matter of the present disclosure includes all novel andnonobvious combinations and subcombinations of the various systems andconfigurations, and other features, functions, and/or propertiesdisclosed herein.

The following claims particularly point out certain combinations andsubcombinations regarded as novel and nonobvious. These claims may referto “an” element or “a first” element or the equivalent thereof. Suchclaims should be understood to include incorporation of one or more suchelements, neither requiring nor excluding two or more such elements.Other combinations and subcombinations of the disclosed features,functions, elements, and/or properties may be claimed through amendmentof the present claims or through presentation of new claims in this or arelated application. Such claims, whether broader, narrower, equal, ordifferent in scope to the original claims, also are regarded as includedwithin the subject matter of the present disclosure.

1. A method for controlling a spark ignited engine having a particulatefilter, comprising: retarding a spark angle at which spark is deliveredto at least one cylinder of a spark ignited engine when an amount ofsoot held by a particulate filter exceeds a threshold and when engineload is less than a threshold level.
 2. The method of claim 1 whereinsaid spark angle at which spark is delivered to said at least onecylinder is advanced from a retarded condition toward minimum spark forbest torque as engine speed increases.
 3. The method of claim 1 whereinsaid spark angle is advanced when a temperature of said particulatefilter reaches a threshold temperature.
 4. The method of claim 1 whereinan amount of fuel delivered to at least a cylinder provides rich orstoichiometric exhaust gases while retarding said spark angle at whichspark is delivered to said at least one cylinder.
 5. The method of claim1 wherein a rate at which air mass flows through said spark ignitedengine is increased in response to retarding said spark angle.
 6. Themethod of claim 5 wherein increasing said mass air flow rate isincreased by adjusting one of a throttle position or cam timing.
 7. Themethod of claim 1 further comprising advancing a spark angle at whichspark is delivered to at least one cylinder of a spark ignited enginewhen a temperature of said particulate filter exceeds a thresholdtemperature.
 8. A method for controlling a spark ignited engine having aparticulate filter, comprising: regenerating a particulate filter of aspark ignited engine; and adjusting at least spark timing of at leastone cylinder of said spark ignited engine to regulate a temperature ofsaid particulate filter above a threshold temperature.
 9. The method ofclaim 8 wherein said spark timing at which spark is delivered to said atleast one cylinder is advanced from a retarded condition toward minimumspark for best torque as a speed of said spark ignited engine increases.10. The method of claim 8 wherein said spark timing is advanced when atemperature of said particulate filter reaches a threshold temperature.11. The method of claim 8 wherein an amount of fuel delivered to atleast one cylinder provides rich or stoichiometric exhaust gases whileretarding said spark timing at which spark is delivered to said at leastone cylinder.
 12. The method of claim 8 wherein a rate at which mass airflows through said spark ignited engine is increased when said sparkangle is retarded.
 13. The method of claim 12 wherein increasing saidmass air flow rate is increased by adjusting one of a throttle positionor cam timing.
 14. The method of claim 8 wherein said adjusting at leastspark timing of at least one cylinder of said spark ignited engineincludes advancing a spark angle at which spark is delivered to at leastone cylinder of a spark ignited engine when a temperature of saidparticulate filter exceeds a threshold temperature.
 15. A method forcontrolling a spark ignited engine having a particulate filter,comprising: regenerating a particulate filter of a spark ignited engine;adjusting at least spark timing of at least one cylinder of said sparkignited engine to regulate a temperature of said particulate filterabove a threshold temperature; and advancing said at least spark timingof said at least one cylinder of said spark ignited engine in responseto an increasing driver demand torque.
 16. The method of claim 15wherein said spark timing is advanced when a temperature of saidparticulate filter reaches a threshold temperature.
 17. The method ofclaim 15 wherein an amount of fuel delivered to at least one cylinderprovides rich or stoichiometric exhaust gases while retarding said sparktiming at which spark is delivered to said at least one cylinder. 18.The method of claim 15 wherein a rate at which mass air flows throughsaid spark ignited engine is increased when said spark angle isretarded.